Here we start restructuring the the schemas YAML such that it
contains a top-level `version` field. In this commit we only modify
the YAML, the build system changes will come later, to avoid
enormous commits.
These schema files are currently generated at build time by
collecting various TUPLE-TREE-YAML comments using the
`extract_yaml.py` script. In this commit we move them to the codebase
but we don't use them yet, this will be done at a later commit for
the sake of readability.
Model classes are now described by a YAML document, which is used to
generate C++ headers containing classes and all the boilerplate
required for YAML serialization/deserialization, usage in
SortedVectors, etc. See the README in include/revng/Model for more
info.